Method for the continuous solution polymerization of methyl methacrylate

ABSTRACT

There is disclosed a process for the continuous polymerization of methyl methacrylate with a lesser cost, thermally polymerizable comonomer such as styrene, in the presence of a large quantity of solvent by introducing the monomers into a continuos loop of recycled solvent and removing the polymerized product therefrom. The polymerization is practiced in a plurality of stages, preferably two stages wherein the bulk of the polymerization is performed in the first stage and the final stage is operated to deplete the initiator and any thermally polymerizable comonomer, and polymerization modifiers from the crude polymerizate, thereby forming a crude polymerizate suitable for devolatilization. The crude polymerizate is preheated and devolatilized in a flash zone. Sufficient pressure is maintained on the crude polymerizate in the devolatilizer preheater to avoid the formation of foam encrustations on the heat exchange surfaces of the devolatilizer preheater and on the surfaces of the transfer lines.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending parentapplication Ser. No. 782,157, filed Oct. 1, 1985, now U.S. Pat. No.4,728,701, which is a continuation of application Ser. No. 533,455,filed Sept. 19, 1983, now abandoned.

BACKGROUND OF THE INVENTION Brief Statement cf the Prior Art

One of the difficulties experienced with the polymerization cf methylmethacrylate is that the polymerization becomes uncontrollable when thecrude polymerizate exceeds a critical polymer solids concentration. Forthis reason, prior polymerization processes have avoided bulkpolymerization and each of the poly(methyl methacrylate) marketed todayis produced by suspension polymerization.

Methyl methacrylate is commonly copolymerized with a limited amount ofcopolymerizable monomers (comonomers) such as methyl acrylate and ethylacrylate. These comonomers stabilize the polymer, particularly againstdepolymerization, which can occur when the polymer chains terminate inunsaturated carbon atoms. The latter results from termination bydisproportionation and, accordingly, an objective of the prior art hasbeen to effect chain termination by free radical coupling rather than bydisproportionation. This is achieved by incorporating in thepolymerization zone a limited quantity of a chain transfer agent such asan alkyl mercaptan, e.g., n-dodecyl mercaptan. The chain termination byfree radical coupling to an alkyl mercaptan, however, forms a mercaptanfree radical which can re-initiate polymerization, resulting in apolymer chain having a mercaptan terminal group.

Poly(methyl methacrylate) is commonly produced by a suspension processbecause of the difficulties experienced with solvent or bulkpolymerization of this monomer. One of the major difficulties limitingthe use of mass polymerization is that the polymerization becomesuncontrollable when the polymerizate reaches a gel condition, typicallyat solids concentrations above a value which commonly is from 30 toabout 40 weight percent. Although solvent polymerization could be usedto obviate this difficulty, prior investigators have not developed anefficient method or the equipment required for the devolatilization oflarge quantities of solvent from the crude polymerizate, preferringinstead to use suspension or block polymerization or other techniques,all of which lack in efficiency and/or product ccnsistency. When solventis present in the crude polymerizate it causes severe foaming duringdevolatilization and this foaming obstructs efficient heat transfer anddevolatilization. Furthermore, the polymer readily discolors ifcontacted with heat transfer surfaces at temperatures in excess ofapproximately 270 degrees C. These characteristics interfere withefficient heat transfer and have, heretofore, precluded the successfulcommercialization of continuous, solution polymerization of acrylateesters such as methyl methacrylate.

BRIEF DESCRIPTION OF THE INVENTION

This invention comprises a process for the continuous masspolymerization of methyl methacrylate, preferably in the presence oflimited amounts of comonomers such as ethyl acrylate and methylacrylate, or in the presence of substantial quantities of lesser costcomonomers such as styrene, alpha-methyl styrene, and maleic anhydride.The polymerization is conducted in a plurality of stages; most of thepolymerization is effected in the early stages, and the latter stagesresidual initiators and/or modifiers. When thermally initiatingcomonomers are present such as styrene and methyl styrene, the latterstage of the polymerization is operated under conditions which willsubstantially deplete these thermally initiating comonomers. Thepolymerization is also conducted in the presence of chain transferagents which enhance the properties of the final product and theseadditives are preferably introduced into all stages. The crudepolymerizate from the final stage is passed to a preheater in thedevolatilization section and is there heated while under a sufficientpressure to maintain an appreciable liquid phase and to suppress theformation of solid, foam encrustations on the heat transfer surfaces ofthe heating equipment. The crude polymerizate is heated sufficiently inthe preheater to supply the heat of vaporization needed for theeffective devolatilization of the solvent, unreacted monomers and lowboiling polymer products. The preheated, crude polymerizate istransferred into a devolatilization zone where the pressure is reducedto flash the solvent, unreacted monomers and lower molecular weightpolymer from the polymer product. The vaporized solvent and monomers arepurified of contaminants, cooled, condensed and recycled on a continuousbasis to the first reaction stage, completing a circulation loop ofrecycled solvent and monomers. The majority, e.g. 75 percent, ofcomonomer such as methyl acrylate, ethyl acrylate or styrene, and chaintransfer additives used in the process are introduced into the firststage with the methyl methacrylate and solvent. The remainder areintroduced into the latter stages of polymerization to ensure that thepolymerization in the latter stage proceeds under conditions which areconducive to formation of a homogeneous copolymer with superiorproperties such as thermal stability and heat distortion temperature.

The polymerization is preferably conducted in continuously stirred tankreactor vessels (CSTR) under sufficient agitation to provide ahomogeneous reacting mass in each of the reactors. The exothermic heatof polymerization is removed, preferably by reflux cooling and, for thispurpose, the reactors are maintained at precisely controlled pressures,to maintain boiling of the polymerizing reaction mass. The vapors arewithdrawn from the reactor, cooled, condensed and returned as a refluxstream. This method of polymerization ensures precise control of thepolymerization conditions such as temperature and permits manufacture ofa product having the desired molecular weight within relatively narrowlimits of variation (molecular weight distribution).

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described with reference to the FIGURE which is aschematic process flow diagram of the method.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention relates to the polymerization of acrylic monomers, inparticular, the polymerization of monomers such as the acrylate andmethacrylate esters of alkanols or halogen substituted alkanols havingfrom 1 to about 18 carbons, e.g., methyl acrylate ethyl acrylate, methylmethacrylate, propyl acrylate, n-butyl methacrylate, n-hexyl acrylate,chloroethyl acrylate, n-octyl methacrylate, stearyl acrylate, alone, orin admixture to produce homo or copolymers. Other common comonomerswhich can be included in amounts from 1 to about 80 weight percentinclude acrylonitrile, styrene, maleic anhydride, alpha-methyl styrene,and mixtures thereof. The process is particularly adapted to theproduction of a copolymer containing from 20 to 80 percent styrene oralpha-methyl styrene, commonly referred to as a styrene acryliccopolymer.

The polymerization is performed in the presence of sufficient solvent toavoid the viscosity of the crude polymerizate rising above a value wherethe polymerization becomes uncontrollable. This is usually expressed interms of solid content and sufficient solvent is used to maintain thesolids content of the crude polymerizate below 50 weight percent,preferably below 40 weight percent. For this purpose, any low boilingpoint solvent can be used such as saturated and aromatic hydrocarbonswhich are exemplified by hexane, heptane, octane, benzene, toluene,xylene, cyclohexane, cyclodecane, isooctane, and mixtures thereof suchas naphtha, etc. Generally solvents having atmospheric boiling pointsfrom about 40 to about 225 degrees C., preferably from about 60 to about150 degrees C., are useful. It is preferred to use a solvent having aboiling point temperature close to that of the major monomer, e.g.,methyl methacrylate, to avoid the necessity of intermediatefractionation of the recycled mixture of monomer and solvent. When thesehave close boiling point temperatures, the mixture has a narrow boilingpoint range, lessening the opportunity for inclusion of contaminates inthe recycle mixture

It is also preferred to conduct the polymerization in the presence of afree radical chain transfer agent to minimize the concentration of thepolymer chains containing terminally unsaturated carbons. For thispurpose, a free radical chain transfer agent is employed. Free radicalchain transfer agents which are useful are compounds which release ahydrogen atom onto the polymer chain, terminating its polymerization ina saturated carbon end group and forming a free radical which canre-initiate polymerization or combine with another free radical to forma stable byproduct. Examples of suitable chain transfer agents includesulfur compounds such as the alkyl and arylalkyl mercaptans having fromabout 5 to about 18 carbons, e.g., amyl mercaptan, heptyl mercaptan,iso-octyl mercaptan, decyl mercaptan, n-dodecyl merraptan, phenylethylmercaptan, hexadecyl mercaptan, stearyl mercaptan, etc.

Other compounds which are useful chain transfer agents are those havinga structure which stabilizes a free radical and include aromatichydrocarbons of 6 to about 18 carbons, and halo, amino or imidosubstituted alkanes or aromatics having from 1 to about 18 carbons.Examples of suitable aromatic hydrocarbons are benzene and C₁ -C₆ alkylbenzenes, e.g., toluene, ethylbenzene, xylene, propyllenzene,isobutylbenzene, isopropyl toluene, diisopropyl benzene, triethylbenzene, etc. Examples of substituted hydrocarbons are compounds havingfrom 1 to about 18 carbons and from 1 to about 6 halo, amino or imidogroups, e.g., carbon tetrachloride, dichloroethane, trichloroethane,difluoro-propane , fluorodichlorobutane, dichloroisopentane,bromocyclohexane, methylamine, isopropylamine, t-butylamine,dodecylamine, 2,4-diaminooctane, cyclopentylamine,Methylcyclohexylamine, aniline, pyridylidene, piperazine, puridine,dimethyl sulfoxide, etc.

A particularly useful selection of a chain transfer agent is one havinga low boiling point, e.g., from 60 to about 150 degrees C. since suchcompounds can also function as the solvent, alone, or in combinationwith any of the aforementioned solvents.

The concentration of the chain transfer agent employed depends on theparticular agent selected. The sulfur compounds or mercaptans are usedat concentration from about 0.1 to about 1.0, preferably from about 0.2to about 0.3 weight percent of the monomer and comonomer feed mixture.The alkylbenzenes, however, are used in much greater excess,particularly since these ingredients can also serve as the solvent forthe process, alone or in admixture with other solvents.

Modifiers can also be included at concentrations from 2 to 50 weightpercent to improve the impact strength of the finished polymer. Theseare elastomers such as ethylene-propylene diamine copolymer (EPDM),polybutadiene, styrene-butadiene copolymer, polyurethane, and ethylenepropylene copolymer rubber (EPR). These modifiers can be included in thefinished polymer, preferably by addition to the polymerization zone. Themodifier can also be blended into the molten polymer or crudepolymerizate during the product finishing steps of the process.

The invention is particularly suited for polymerization of methylmethacrylate, preferably in the presence of limited amounts of acomonomer such as ethyl acrylate or methyl acrylate in amounts from 0.1to about 12, and preferably from 1 to about 6 weight percent of theresulting copolymer. The process, however, can also be used for thecopolymerization of methyl methacrylate with lesser cost comonomers suchas styrene or alpha-methyl styrene, in which the lesser cost comonomersare from 5 to 80, preferably from 40 to 60 percent of the polymer.

The polymerization of the aforementioned monomers is initiated with afree radical initiator and, for this purpose, any of a number of freeradical precursors can be used as initiators. Examples of usefulinitiators are: dibenzoyl peroxide, dicumyl peroxide,2,2,'-azo(bis)isobutylnitrile, 2,2,'azobis(dimethylvaleronitrile),diethyl peroxide, distearyl peroxide, t-butyl peroxide,di(2,4-dichlorobenzoyl) peroxide, diacetyl peroxide, t-butylperbenzoate, t-amyl peroctoate, 1,1-di(t-butylperoxy)cyclohexane,di(t-butyl)peroxide, dicumyl peroxide, etc. Of the aforementioned,2,2'azo(bis)isobutylnitrile is preferred. The initiator can be employedat concentrations in the monomer feed mixture from about 0.01 to 1.0weight percent, preferably from about 0.03 to about 0.5 weight percent,and most preferably from 0.07 to 0.10 weight percent.

Other useful additive for the process include peroxy free radicalscavengers to preclude any formation of polymer that would include anoxy group. The presence of any significant amounts of polymer containingan oxy group is undesirable since such polymers have poor weather andthermal resistance and readily discolor. Significant amounts ofoxysubstituted polymer can be precluded by including as an additive inthe process a limited quantity of a peroxy free radical precursor suchas: hindered phenol antioxidants,tetrakis(methylene(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate))methane,thiodiethylene bis(3,5-di-tert-buty-4-hydroxyl)hyudrocinnamate,1,6-hexamethylene bis(3,5-di-tert-butyl-4-hydroxy-hydrocinnamate,di-t-butyl-p-cresol, octadecyl 3-(3',5,'-di-tert-butyl-4'hydroxyphenyl)propionate, tris(3,5-di-t-butyl-4-hydroxybenzyl)iso-cyanurate,2,2'-methylene bis(4-methyl-6-t-butylphenol), and 3:1 condensate of3-methyl-6-t-butylphenol with crotonaldehyde.

Of the aforementioned, octadecyl3-(3;,5;-di-tert-butyl-4'-hydroxyphenyl)propionate andtris(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate are the preferredadditives as they do not interfere with the polymerization. Theaforementioned peroxy free radical scavengers are employed atconcentrations from about 0.01 to about 0.5, preferably 0.1 to about0.2, weight percent, based on the monomer feed mixture.

Referring row to the FIGURE, the process is practiced in two or morecontinuously stirred tank reactors 10 and 20. Each reactor comprises asteel vessel with a centrally disposed propeller shaft 11 and 21extending from a superimposed propeller drive and motor assembly 12 and22. Each propeller shaft such as 11 supports one or more propellersformed of a can also function as the solvent, alone, or in plurality ofradial blades 13 which can be entirely radial, i.e., having no axialpitch, or can have an axial pitch, e.g., from about 5 to 45 degrees, asshown in the side view of propeller blade 14. The propellers intimatelyadmix the polymerization medium to form a homogeneous mass in eachreactor.

Each tank reactor has a bottom discharge nozzle that, preferably,discharges directly into a closely coupled gear pump 15 and 25 such ascommercially available polymer gear pumps. Each tank reactor has aninlet line 17 and 27 for introduction of the polymerizable reactants.The recycle mixture comprising chiefly solvent is introduced throughsolvent recycle line 30 and the fresh feed mixture comprising the methylmethacrylate and limited allowance of comonomers, chain transfer agent,peroxide radical scavenger etc. is introduced through line 32.

The crude polymerizate is transferred from the first tank reactor 10through polymer gear pump 15 and transfer line 34 to the second tankreactor 20. Additional quantities of comonomers and, optionally, chaintransfer agents are also introduced into tank reactor 20 through thedelayed addition line 36.

The exothermic heat of polymerization is removed from the reactors 10and 20 by reflux cooling. To this end, the tank reactors 10 and 20 aretotally enclosed and are maintained at predetermined pressure. Whensubatmospheric pressures are required, a vacuum system is employed,generally indicated at 40. Each tank reactor has a vapor withdrawal line18 and 28 which discharges into a shell and tube condenser 50 and 51.Cooling water is supplied to these condensers through lines 54 and 56and the condensed reflux liquid is supplied to their respective reactorsthrough reflux lines 58 and 59.

The heat exchangers 50 and 51 are connected to the pressure controlsystem by vapor lines 60 and 61 which are provided with pressure controlvalves 61 and 63. These valves are controlled by a suitable pressurecontroller which maintains a predetermined pressure in each reactor,thereby providing a precise control of the pressure in their respectivetank reactors 10 and 20, and thus achieving a precise temperaturecontrol within these tank reactors. In practice, the temperatures withinthe first and second tank reactors can be controlled within a toleranceof 0.5 degrees C., a precision which gives very close control over themolecular weight and molecular weight distribution of the polymerproduced in these reactors.

As previously mentioned, most of the polymerization is performed in thefirst tank reactor 10. Typically, from 20 to 95 percent, preferably 65to 95, of the conversion of the monomer achieved in the process isaccomplished in the first tank reactor. Generally, the temperaturewithin this reactor is from 60 degrees to about 130 degrees C. and theparticular temperature chosen is selected for the molecular weight andother properties desired in the final polymer product.

The second tank reactor serves to complete the polymerization and todeplete the crude polymerizate of any residual or unconsumed initiator.When thermally polymerizable comonomers such as styrene and alphamethylstyrene are copolymerized with the methyl methacrylate, the second tankreactor is operated under conditions to also deplete these comonomersfrom the crude polymerizate. This is accomplished by providing anextended residence time to ensure that substantially all the initiatorand thermally polymerizable comonomer is consumed. For optimumproperties of the final polymer, additional quantities of thecomonomers, such as methyl methacrylate, ethyl acrylate or methylacrylate, are introduced into the second tank reactor together with theintermediate polymerizate transferred from the first tank reactor 10.Approximately 5 to about 50 percent, preferably about 25 percent of thetotal of the comonomers which are used in the process are introducedinto the second tank reactor 20 through line 36. As previouslymentioned, additional quantities of the chain transfer additive, e.g.,n-dodecyl mercaptan, are also introduced into the second tank reactor20. Approximately 5 to about 50 percent of the total chain transferagent employed in the process, preferably about 25 percent, isintroduced into the second tank reactor through line 36.

The range cf temperatures for this second tank reactor 20 is also fromabout 60 to about 130 degrees C.

The crude polymerizate withdrawn from the second tank reactor 20 istransferred by polymer pump 25 through transfer line 38 to thedevolatilizer preheater 70. The crude polymerizate is preferably passedthrough the tubes of a shell and tube heat exchanger and is raised to astock temperature between about 220 -260 degrees C. in this exchanger byheat transfer with a heating fluid, e.g., a hot oil stream which isintroduced into the shell side of the exchanger through line 71. Thecrude polymerizate which has been heated in heat exchanger 70 istransferred through transfer line 39 to the flash vessel 76 in thedevolatilizer section. The transfer line 39 is provided with a backpressure control valve 72 which is responsive to the discharge pressureof polymer gear pump 25. The pressure which is maintained on the crudepolymerizate is sufficient to maintain a mixed, two-phase system ofliquid and vapors. In practice, a substantial portion of the solvent ismaintained in liquid phase in preheater 70, thereby avoiding theformation of foam encrustations on the surfaces of the heat exchangerand permitting efficient heat transfer in the devolatilizer preheater.When excessive vaporization occurs in the heat exchanger, the crudepolymer can become cooled to or below the temperature of its meltingrange, resulting in formation of a solid phase which quickly forms foamencrustations on the heat transfer surfaces. This is avoided bymaintaining sufficient back pressure on the crude polymerizate inpreheater 70.

The necessary back pressure for a crude polymerizate having anycombination of monomer, comonomers, and solvent can be determinedexperimentally by heating a sample of the crude polymerizate to theinlet temperature to the preheater in a laboratory pressure bomb whilemaintaining sufficient pressure on the sample to prevert any substantialvaporization. The pressure is then slowly released from the sample whileobserving the liquid phase of the sax to determine the pressure at whichincipient solidification occurs. This pressure is the minimum pressureto maintain at the inlet to the preheater 70.

Care must also be exercised to avoid heating the crude polymerizate totemperatures in excess of approximately 270 degrees C. as such elevatedtemperatures discolor the product. Efficient devolatilization, however,requires that the crude polymerizate be heated to about 240-250 degreesC. and, accordingly, the temperature limit between a successfulefficient operation and an operation producing discolored polymer isvery narrow.

The crude polymerizate is flashed into devolatilizer vessel 76 which ismaintained at a subatmospheric pressure sufficient to strip nearly allof the solvent, unreacted monomer, comonomer and the low boiling polymerbyproduct from the finished polymer product. Preferably a spray sparger78 is used to ensure intimate dispersion of the polymerizate into finesheets or droplets for efficient devolatilization. Typically, thedevolatilizer is maintained at an absolute pressure of from 10 to about150 mm Hg., preferably at about 50 mm Hg. The vapors are removed fromvessel 76 through a nozzle in its top dome and the vapors are passed byline 80 to recycle still 81, which is a column having two zones ofpacking 83 and 85 and a subjacent reboiler 87. The hot vapors areintroduced beneath the lower packed zone 85 and are partially condensedby contact with recycled condensate from lines 86 and 91. The rate ofrefluxed condensate through line 86 is controlled by valve 79 tomaintain a preselected liquid level in reboiler 87. Solvent some monomerand the low boiling polymer products accumulate in reboiler 87 and ableed stream of these is removed at 93.

The vapors from recycle still 81 are passed by line 95 to the recyclecondenser 82. The recycle condenser 82 is a shell and tube heatexchanger and the solvent vapors are condensed and collected in recycleaccumulator vessel 84. The noncondensibles are passed by line 97 to thevacuum system 40 through control valve 99. A portion of the condensedsolvent is returned to the recycle still as reflux through line 86 whilethe remainder of the condensed solvent is recycled to the processthrough line 30, completing the solvent loop of the process. Thefinished polymer, which typically has a residual monomer and solventcontent less than about 1.0 weight percent, preferably less than 0.1weight percent, is withdrawn from the bottom of the devolatilizer vessel76 through polymer pump 75 and is passed to the finishing treatment. Inthe finishing treatment, the polymer is blended with additives which areintroduced by line 89 through injector nozzle 88 and intimately mixed bypassage through static mixer 90, a commercially available unit having aplurality of successive, oppositely curved, stationary blades. Theaforementioned elastomeric modifier can be added at this point, or otheradditives such as ultra violet stabilizers, antioxidants, internallubricants/processing aids, thermal stabilizers, dyes/opticalbrighteners and plasticizers can be added at conventionalconcentrations. The polymer is forced through a screen 92 to removeparticulate contaminants and the filtered polymer is then extrudedthrough die 94 in the form of a plurality of strands of molten polymerand the strands of extruded polymer are passed through water bath 96 tosolidify the polymer. The solidified polymer is then passed throughwater stripper 98 which removes residual moisture and serves to furthercool the polymer strands 100 that are passed to a pelletizing station102 where the polymer strands are cut into pellets 104 suitable for usein plastics fabricating equipment such as extrusion and injectionmolding equipment.

We claim:
 1. A continuous solution process for the polymerization ofmethyl methacrylate monomer with thermally polymerizable comonomerscomprising styrene, alpha-methyl styrene, and maleic anhydride whichcomprises:(a) introducing methyl methacrylate and from 5 to 80 percentof the thermally polymerizable comonomer, initiator and chain transferagent into admixture with solvent to provide a polymerization mediumcontaining not less than 40 weight percent solvent; (b) maintaining saidpolymerization medium under polymerization conditions, in at least onepolymerization reactor comprising a temperature from 60 to about 130degrees C. and sufficient residence time to effect 20 to 95 percent ofthe total conversion of said methyl methacylate and comonomer and form acrude intermediate polymerizate; (c) withdrawing said crude,intermediate polymerizate and passing it to at least one succeedingpolymerization reactor, including a final polymerization reactor; (d)stirring said crude intermediate polymerizate in said succeedingpolymerization reactor, including said final polymerization reactor, tomaintain said polymerizable intimately admixed while maintainingpolymerization conditions therein comprising a temperature from 60 toabout 130 degrees C., and sufficient residence time to essentiallydeplete the initiator and the thermally polymerizable comonomer in saidpolymerizate, thereby forming a crude polymerizate containing solvent,unreacted monomer and comonomer and less than about 50 weight percentpolymer, and pumping said crude polymerizate from said finalpolymerization reactor to a devolatilizer preheater; (e) heating saidcrude polymerizate in said devolatilizer preheater to a temperaturebetween 200 degrees and 270 degrees C., withdrawing the preheated crudepolymerizate from said preheater and passing it through a transfer lineto a devolatilizer and spraying said preheated polymerizate into saiddevolatilizer and, flashing the polymerizate therein to remove avolatile stream comprising solvent, unreacted monomer and comonomer, andoligomers from a molten polymer; and (f) maintaining a pressure on thecrude polymerizate in said transfer line and said devolatilizerpreheater which permits vaporization therein to form a mixed two-phasesystem of vapor and liquid, which pressure is sufficient to preventincipient solidification in the liquid phase, thereby avoiding formationof foam encrustations on the heat exchange surfaces of said preheater,while heating said polymerizate sufficiently in said preheater toprovide a preheated crude polymerizate with sufficient heat to supplythe heat of vaporization necessary to produce said molten polymer andpumping said molten polymer from said devolatilizer.
 2. The process ofclaim 1 wherein said comonomer is styrene.
 3. The process of claim 2wherein said first reactor is provided with a mixture of from 20 to 95percent methyl methacrylate and from 80 to 5 percent styrene.
 4. Theprocess of claim 1 wherein said solvent is toluene.
 5. The process ofclaim 1 wherein said solvent is selected from the class consisting ofbenzene, alkyl benzenes with 6-10 total carbons, and mixtures thereof.6. The process of claim 1 wherein said chain transfer agent is n-dodecylmercaptan.
 7. The process of claim 1 wherein said polymerization mediumis intimately admixed to form a thermally and chemically homogeneousmass in all zones of polymerization.
 8. The process of claim 1 wherein aportion of the vapors formed in the first polymerization zone iswithdrawn, cooled and condensed and the resultant condensate is returnedto said first polymerization zone to maintain said temperature.
 9. Theprocess of claim 8 including the application of a vacuum to said firstpolymerization zone and maintaining the pressure in said zones bypassing vapors withdrawn from said zones through throttling valves. 10.The process of claim 1 wherein said crude polymerizate is heated byindirect heat exchange with heated heat transfer surfaces in saidvolatilizer preheater.
 11. The process of claim 10 wherein said crudepolymerizate is maintained under pressure during its heating to preventformation of a solid phase during said heating step, thereby avoidingthe formation of solid encrustations on said heat transfer surfaces. 12.The process of claim 1 including the step of adding from 5 to about 30percent of the methyl methacrylate used in said process to said crude,intermediate polymerizate in said at least one succeeding polymerizationzone including said last polymerization zone.
 13. The process of claim12 including the step of adding from 5 to about 30 percent of the chaintransfer agent used in said process to said crude, intermediatepolymerizate in said at least one succeeding polymerization zoneincluding said last polymerization zone.
 14. The process of claim 1including the step of adding to the polymerization medium a peroxy freeradical scavenger in an amount from 0.05 to 0.5 weight percent based onsaid medium.
 15. The process of claim including the steps of:(g)condensing said volatile stream to obtain a condensate comprising saidsolvent and unreacted methyl methacrylate; and (h) recycling saidcondensate to said first polymerization reactor as the source of solventtherein.